Various polymerization mechanisms are used for curing polymerizable materials such as dental materials. For materials which are based on (meth)acrylates, the curing takes place via a radical polymerization, wherein radical formation and thus curing can be initiated thermally, chemically, or by light. Thermal curing is used predominantly for prosthesis materials, but does not come into consideration for a curing under oral conditions. The redox initiator systems used for chemical initiation consist at least of two components, usually a peroxide and a reductant. Due to the high reaction rate of peroxide and reductant, the two components may be combined only shortly before the curing of the dental material, and only a short processing time is available. Moreover, in the case of highly-filled filling composites in particular, it is possible only with difficulty to mix the so-called initiator paste with the accelerator paste without introducing air bubbles, with the result that photoinitiators are mainly used for this. A further disadvantage is the poor storage stability of peroxides used as initiators. A major disadvantage of light-curing dental materials is the limited through-curing depth, in particular of pigmented composites, with the result that they must be cured in layers, which is time-consuming.
In so-called frontal polymerization (FP), a reaction zone is produced which goes from the sample surface through the whole reaction mixture and thus results in a polymerized product. A. Khan, J. A. Pojman, Trends in Polymer Science, 4 (1996) 253-257, describe the frontal polymerization of n-butyl acrylate, in which the reaction is started on the surface by heat. The reaction heat produced during the polymerization of the surface layer triggers the polymerization in the adjacent layer, and a polymerization front thus forms which goes through a polymerization mixture at a rate of approximately 1 cm per minute. Temperatures of up to 290° C. are reached in the reaction zone. As the high temperatures can lead to the evaporation of the monomers and thus to blistering, the reaction is carried out under a pressure of >15×105 Pa.
According to Washington and Steinbock, Polymer News, 2003, Vol. 28, 303-310, high temperatures can lead to a decomposition of the initiator (“initiator burnout”) and thus stop the polymerization front.
Nason et al., Macromolecules 38 (2005) 5506-5512, disclose the UV-light-induced frontal polymerization of multifunctional (meth)acrylates. At the polymerization front, temperatures of over 200° C. were measured which are not suitable for intraoral dental use.
U.S. Pat. No. 4,222,835, which is hereby incorporated by reference in its entirety, discloses compositions curable by frontal polymerization which are intended to be suitable for coating glass fibres or other objects.
U.S. Pat. Nos. 6,057,406 and 6,313,237, which are hereby incorporated by reference in their entirety, disclose the production of polymeric gradient materials by thermally initiated frontal polymerization.
A mortar curable by frontal polymerization which is intended to be suitable in particular for fastening tie bars and reinforcing iron in boreholes is known from U.S. Pat. No. 6,533,503, which is hereby incorporated by reference in its entirety.